The WebDriver tool instance will setup the specific WebDriver target to test against: a web browser or a mobile phone.
It also provides non user interface methods for things specific to WebDriver which are not supported by the Windows Automation Tool, such as navigating to an URL or reloading the page.
Because the WebDriver tool is also a Remote Windows Automation Tool, it is always used with an instance of the Windows Automation Tool which then references the WebDriver Tool.
